The present invention relates to a vacuum switchgear and more particularly to a vacuum switchgear, in which a switch having a stationary electrode and a movable electrode is enclosed in a vacuum container that requires grounding.
As a conventional vacuum switchgear, for example, there is a construction, in which a vacuum container accommodates a main circuit switching unit for connecting and disconnecting a main circuit conductor to and from a load side conductor or a bus side conductor and a main circuit/earth selecting unit for connecting the main circuit conductor to and from the bus side conductor or the load side conductor and an earthing conductor and in which the vacuum container is formed of a grounded metal or an insulating material with a grounded layer on its surface. This vacuum container accommodates the main circuit conductor and a part of each of the bus side conductor, the earthing conductor and the load side conductor. Further, the vacuum container also has an air terminal unit that respectively connects in air those portions of the bus side conductor, the earthing conductor and the load side conductor projecting from the vacuum container to the bus side, the earth side and the load side. On the outside of the vacuum container, drive mechanisms are installed to operate the main circuit switching unit and the main circuit/earthing selecting unit. This construction is seen in JP-A-2001-346306, page 4-6 and FIG. 4, for instance.
By integrating an interrupting function, a disconnecting function and an earthing function in the single earthed vacuum container, the interruption and insulation performance can be enhanced, allowing the devices to be made compact. During assembly, the vacuum container with the integrated functions serves as one component. There is therefore an advantage that the number of components can be reduced and the reliability improved. Further, by earthing the vacuum container an inspection can be performed online.
In the conventional vacuum switchgear, particulate foreign matters of a few micron meters may be formed by the open-close operation of the switching unit or during the manufacturing process. If these particulates reach a high electric field area in vacuum, a dielectric breakdown may occur even at service voltage. More specifically, an intermediate shield is provided around the main circuit switching unit among the switching units in the vacuum switchgear to shield an arc produced during the open-close operation, and a dielectric breakdown is prevented from being triggered by an arc. However, the other conductors than the main circuit switching unit also provide electric field concentrated regions but are not shielded. If foreign particles enter the electric field concentrated regions, therefore, they can conduct electricity between the conductors in the field concentrated regions and the grounded vacuum container, resulting in a dielectric breakdown. This dielectric breakdown may occur even when the dielectric strength of devices is verified in a withstand voltage test to be several times the service voltage. When this dielectric breakdown is detected as a ground fault by a protective control system in the power system, a circuit breaker may be tripped, resulting in a power system failure. To prevent the dielectric breakdown the field intensity of the electric field concentrated regions has to be reduced, and therefore the sizes of the devices require increasing to lengthen the insulation distances.